Hello. If someone is robbed of their computer with all the log-in passwords on web pages stored in their browser (in my case Firefox), you should change them. What would be the best solution for this? (Logging into your profile on each page and changing it would be slow and laborious and I don't know if there can be any more automated way)

Slow and laborious would be the only way, if it were too easy to log in to these accounts and change the password then there would be no (or very little) security involved which would defeat the point of using usernames/passwords in the first place.

If you are worried about your passwords being stolen then don't have the browser store them.Write them on a piece of paper. Then the only thing you have to worry about is someone breaking into your house and stealing them or your house catches on fire.

If you have lost or forgotten your master password or you want to disable the feature, you can reset the master password. Resetting the master password will remove all stored password information. Upon resetting, you will lose all the stored information in the Password Manager, as this is a built-in security feature to prevent people from simply resetting your Master Password to gain access to your passwords.